1. Field of the Invention
This invention relates to an antenna module that receives electric waves of an analogue terrestrial broadcasting and a digital terrestrial television broadcasting and, in particular, to an antenna module that is capable of changing a frequency characteristic according to a received frequency channel so that a reception characteristic can be enhanced. Also, this invention relates to a radio device and a mobile radio terminal comprising the antenna module built-in.
2. Description of the Related Art
Conventionally, antenna modules of various types are known. For example, a mono-pole antenna is shown in JP-A-2001-251131 as conventional antenna receives electric waves of lower frequency used in an analogue terrestrial broadcasting and a digital terrestrial television broadcasting in comparison with electric waves used in a mobile phone.
This sort of antenna module is capable of responding to a wide band required. However, since it comprises a low Q (queue), there is a problem that it comprises a low reception sensitivity. Also, since a wave length of an electric wave received by the antenna is 64 cm, there is a problem that it is difficult for the antenna to be built in a mobile terminal comprising a dimension of about 20 cm at a maximum such as a mobile phone, PDA (Personal Digital Assistants), and a receiving device.
A mobile radio terminal comprising a high performance has been developed. For example, hereinafter it is expected for a mobile phone to advance to a multi-functional phone comprising a receiving function or a radio communication function such as a television broadcasting, a radio broadcasting, a radio LAN (Local Area Network) in addition to a phone function. Each of these radio systems is allocated a different frequency band respectively and uses a channel determined by a frequency division of the frequency band. For example, a digital terrestrial television broadcasting is allocated a wide frequency band being an UHF band of 470 MHz to 770 MHz, each broadcasting station is allocated a narrow band divided the wide frequency band at even intervals of 6 MHz, and it is necessary for viewers to select a frequency of a desired channel for viewing a television broadcasting.
It is preferable to avoid receiving all the wide frequency bands used by each radio system with a single antenna in view of suppressing an interfering wave, and it is expected to achieve a high performance in view of a sensitivity of the antenna by selecting a narrow frequency band for sending and receiving. For these reasons, generally a different antenna is provided for each radio system respectively. And also, a television receiving system may comprise a mechanism that unnecessary frequency channels other than desired frequency channels are removed by a tracking filter circuit and so on disposed at a latter part of the antenna.
On the other hand, since the mobile radio terminal is required to be miniaturized in view of improving portability, each antenna is also required to be miniaturized. In this background, a mobile radio terminal is shown in JP-A-2000-36702, the mobile radio terminal comprising a tuning type antenna controlling an impedance matching central frequency by tuning the frequency in a telephone call frequency.
Further, in this specification “a radio device” means a device conducting a telecommunication function itself of sending and receiving an electric wave and “a mobile radio terminal” means a mobile phone, a PDA, a downsized computer comprising a CPU, a camera, a microphone, a speaker, an indicator and so on.
It is considered that in case that all the frequency bands used by each radio system are inputted to a receiving circuit disposed at a latter part of the antenna, a receiving electric power is consumed too much so that a distortion generates in a latter circuit and a receiving characteristic deteriorates.
Further, it is considered that all the frequency bands are inputted to the receiving circuit so that an undesired wave interferes with a desired wave as an interfering wave and a receiving characteristic deteriorates.
Furthermore, it is considered that in case of the tuning type antenna shown in the JP-A-2001-251131 a signal line for controlling the antenna is outputted from a logical circuit part so that a length of the signal line becomes too long. Therefore, it becomes subject to a noise and so on.